The invention relates to a method for operating an electric motor and to a method for operating an electronic power steering system of a motor vehicle. The invention additionally relates to an electronic power steering system of a motor vehicle and to an electronic control unit of such an electronic power steering system.
A steering system is known from DE 100 13 711 A1, which includes a servo drive for torque assistance (power steering). The degree of torque assistance in the described steering system is dependent on the vehicle speed. While the torque assistance is greater at a low vehicle speed (for example, so as to be able to operate the steering wheel without great effort when maneuvering the vehicle), the torque assistance is lowered at higher speeds (for example so as to increase driving stability when driving on a highway). The servo drive can be operated electrically or hydraulically. In addition to the servo drive, the steering system described in DE 100 13 711 A1 also comprises superimposition means, which generate an additional angle and superimpose this additional angle on the steering wheel angle predetermined by the steering handle.
It is known to operate electric motors as electric servo drives in electronic power steering systems or power steering devices. This is generally carried out by way of output stage driver circuits and inverters. The electric motors used typically comprise high-performance magnets. However, the magnetic properties of such high-performance permanent magnets are highly temperature-dependent. The permanent magnets or magnetic elements notably can lose the magnetic properties thereof in a manner that is entirely or partially irreversible if they are exposed to corresponding temperatures and opposing magnetic fields generated by electrical current. The magnets in electric motors must therefore be designed for extreme or maximum ambient requirements in terms of temperatures or opposing fields. The temperatures of the magnetic elements can usually not be directly measured during the operation of the electronic power steering system. Rare earth magnets are demagnetized more quickly at high temperatures, while ferrite magnets are demagnetized more quickly at cold or low temperatures.
Proceeding from this, it is the object of the present invention to provide a method of the type mentioned above, in which magnetic elements of the electric motor are effectively protected from temperature-dependent demagnetization during operation.